1. Field of the Invention
The future of composites is now well established. Composite materials such as glass-epoxy, graphite-epoxy, boron-epoxy, etc., are replacing metal and other conventional materials in the manufacture of aircraft at a rapidly accelerating rate, and also offer a largely untapped potential in an ever-expanding variety of products.
As an example, in the manufacture of the forward section of the Northrop YF-17 aircraft, the use of graphite composites reduced weight by 40%, and costs by 30% compared to the all-metal design for the same assembly. Moreover, the all-metal fuselage section of the aircraft required 1300 parts and 20 subassemblies whereas only 260 parts and 10 subassemblies were required in an identical section fabricated of composites.
One troublesome problem that has been encountered in the manufacture of composite assemblies such as, for example, laminated panel sections, is the intrusion of moisture into the cured matrix. Since moisture in any measurable amount may destroy the integrity of the structure, it is of the utmost importance to detect its presence so that remedial steps can be taken to correct the problem.
2. Description of the Prior Art
There are known methods and apparatus for detecting the presence of moisture in the interior of various materials and compostions, and the exterior as well.
U.S. Pat. No. 2,359,787 (Peters et al), for example, discloses a prism mounted in an aircraft wing with one of its faces flush with an opening in the wing skin and exposed to light. The variation of the reflection of light on the prism face is sensed by a photoelectric cell to detect the presence of ice on the wing. U.S. Pat. No. 3,540,025 (Levin et al) discloses a similar ice detector which also automatically actuates a heating system to melt the ice, when its presence is detected.
Apparatus employing prisms and glass rods are described in U.S. Pat. Nos. 3,528,278 (Sterling) and 3,433,570 (Hansen), respectively, for detecting the presence of vapor or other gaseous media in a confined environment.
Other prior known apparatus such as hygrometers for detecting the presence of water in diverse materials including soil, are disclosed in U.S. Pat. Nos. 2,740,032 (Bouyoucos), 3,550,439 (Hollies et al), and 3,680,364 (Carrier).
None of the prior methods or means, however, are suitable for detecting the presence of moisture in laminated composite structures, primarily from the standpoint of adaptability, complexity, and cost.
while it is possible that more pertinent prior art exists, Applicants' search is believed to have been conducted with a conscientious effort to locate and evaluate the most relevent art available at the time, but the statement is not to be construed as a representation that no more pertinent art exists.